553 research outputs found
Entropy generation analysis of thermally developing forced convection in fluid-saturated porous medium
Entropy generation for thermally developing forced convection in a porous medium bounded by two isothermal parallel plates is investigated analytically on the basis of the Darcy flow model where the viscous dissipation effects had also been taken into account. A parametric study showed that decreasing the group parameter and the PĂ©clet number increases the entropy generation while for the Brinkman number the converse is true. Heatline visualization technique is applied with an emphasis on B
Comments on âViscous-dissipation effects on the heat transfer in a Poiseuille flowâ by O. Aydin and M. Avci
This note comments on an article by Aydin and Avci [1]; the paper treats the same problem as that studied analytically by Ou and Cheng [2] and numerically by Hwang et al. [3], namely the effects of viscous- dissipation on forced convection in a parallel plate channel
Strong CP, Up-Quark Mass, and the Randall-Sundrum Microscope
In the Randall-Sundrum model, setting the ratio of up and down quark masses
, relevant to the strong CP problem, does not require chiral
symmetry or fine-tuning, due to exponential bulk fermion profiles. We point out
that such geometric suppression of the mass of a fermion magnifies the masses
of its corresponding Kaluza-Klein (KK) states. In this sense, these KK states
act as "microscopes" for probing light quark and lepton masses. In simple
realizations, this hypothesis can be testable at future colliders, like the
LHC, by measuring the spectrum of level-1 KK fermions. The microscope can then
provide an experimental test for the vanishing of in the ultraviolet,
independently of non-perturbative determinations, by lattice simulations or
other means, at hadronic scales. We also briefly comment on application of our
microscope idea to other fermions, such as the electron and neutrinos.Comment: 7 pages. New discussions and references added. Main previous
conclusions unchange
Entropy generation for natural convection by heated partitions in a cavity
Entropy generation for natural convection in a partitioned cavity, with adiabatic horizontal and isothermally cooled vertical walls, is studied numerically by both a FORTRAN code and the commercially available CFD-ACE software. Effects of the Rayleigh number, the position of the heated partition, and the dimensionless temperature difference on the local and average entropy generation rate are investigated. Proper scale analysis of the problem showed that, while fluid friction term has nearly no contribution to entropy production, the heat transfer irreversibility increases monotonically with the Nusselt number and the dimensionless temperature difference
Entropy generation for forced convection in a porous saturated circular tube with uniform wall temperature
A numerical study is reported to investigate both the First and the Second Law of Thermodynamics for thermally developing forced convection in a circular tube filled by a saturated porous medium, with uniform wall temperature, and with the effects of viscous dissipation included. A theoretical analysis is also presented to study the problem for the asymptotic region applying the perturbation solution of the Brinkman momentum equation reported by Hooman and Kani [1]. Expressions are reported for the temperature profile, the Nusselt number, the Bejan number, and the dimensionless entropy generation rate in the asymptotic region. Numerical results are found to be in good agreement with theoretical counterparts
Addition of 24âhour heart rate variability parameters to the Cardiovascular Health Study stroke risk score and prediction of incident stroke: The Cardiovascular Health Study
Background Heart rate variability (HRV) characterizes cardiac autonomic functioning. The association of HRV with stroke is uncertain. We examined whether 24âhour HRV added predictive value to the Cardiovascular Health Study clinical stroke risk score (CHSâSCORE), previously developed at the baseline examination. Methods and Results N=884 strokeâfree CHS participants (age 75.3±4.6), with 24âhour Holters adequate for HRV analysis at the 1994â1995 examination, had 68 strokes over â€8 year followâup (median 7.3 [interquartile range 7.1â7.6] years). The value of adding HRV to the CHSâSCORE was assessed with stepwise Cox regression analysis. The CHSâSCORE predicted incident stroke (HR=1.06 per unit increment, P=0.005). Two HRV parameters, decreased coefficient of variance of NN intervals (CV%, P=0.031) and decreased power law slope (SLOPE, P=0.033) also entered the model, but these did not significantly improve the câstatistic (P=0.47). In a secondary analysis, dichotomization of CV% (LOWCV% â€12.8%) was found to maximally stratify higherârisk participants after adjustment for CHSâSCORE. Similarly, dichotomizing SLOPE (LOWSLOPE <â1.4) maximally stratified higherârisk participants. When these HRV categories were combined (eg, HIGHCV% with HIGHSLOPE), the câstatistic for the model with the CHSâSCORE and combined HRV categories was 0.68, significantly higher than 0.61 for the CHSâSCORE alone (P=0.02). Conclusions In this sample of older adults, 2 HRV parameters, CV% and power law slope, emerged as significantly associated with incident stroke when added to a validated clinical risk score. After each parameter was dichotomized based on its optimal cut point in this sample, their composite significantly improved prediction of incident stroke during â€8âyear followâup. These findings will require validation in separate, larger cohorts. Keywords: autonomic nervous system, clinical stroke risk model, heart rate variability, prediction, predictors, risk prediction, risk stratification, strok
Detecting solar axions using Earth's magnetic field
We show that solar axion conversion to photons in the Earth's magnetosphere
can produce an x-ray flux, with average energy \sim 4 keV, which is measurable
on the dark side of the Earth. The smallness of the Earth's magnetic field is
compensated by a large magnetized volume. For axion masses < 10^{-4} eV, a
low-Earth-orbit x-ray detector with an effective area of 10^4 cm^2, pointed at
the solar core, can probe the photon-axion coupling down to 10^{-11} GeV^{-1},
in one year. Thus, the sensitivity of this new approach will be an order of
magnitude beyond current laboratory limits.Comment: 3 pages, 1 figure, typos corrected, references adde
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Early Recognition of Burn- and Trauma-Related Acute Kidney Injury: A Pilot Comparison of Machine Learning Techniques.
Severely burned and non-burned trauma patients are at risk for acute kidney injury (AKI). The study objective was to assess the theoretical performance of artificial intelligence (AI)/machine learning (ML) algorithms to augment AKI recognition using the novel biomarker, neutrophil gelatinase associated lipocalin (NGAL), combined with contemporary biomarkers such as N-terminal pro B-type natriuretic peptide (NT-proBNP), urine output (UOP), and plasma creatinine. Machine learning approaches including logistic regression (LR), k-nearest neighbor (k-NN), support vector machine (SVM), random forest (RF), and deep neural networks (DNN) were used in this study. The AI/ML algorithm helped predict AKI 61.8 (32.5) hours faster than the Kidney Disease and Improving Global Disease Outcomes (KDIGO) criteria for burn and non-burned trauma patients. NGAL was analytically superior to traditional AKI biomarkers such as creatinine and UOP. With ML, the AKI predictive capability of NGAL was further enhanced when combined with NT-proBNP or creatinine. The use of AI/ML could be employed with NGAL to accelerate detection of AKI in at-risk burn and non-burned trauma patients
Effects of temperature-dependent viscosity variation on entropy generation, heat and fluid flow through a porous-saturated duct of rectangular cross-section
Effect of temperature-dependent viscosity on fully developed forced convection in a duct of rectangular cross-section occupied by a fluid-saturated porous medium is investigated analytically. The Darcy flow model is applied and the viscosity-temperature relation is assumed to be an inverse-linear one. The case of uniform heat flux on the walls, i.e. the H boundary condition in the terminology of Kays and Crawford, is treated. For the case of a fluid whose viscosity decreases with temperature, it is found that the effect of the variation is to increase the Nusselt number for heated walls. Having found the velocity and the temperature distribution, the second law of thermodynamics is invoked to find the local and average entropy generation rate. Expressions for the entropy generation rate, the Bejan number, the heat transfer irreversibility, and the fluid flow irreversibility are presented in terms of the Brinkman number, the PĂ©clet number, the viscosity variation number, the dimensionless wall heat flux, and the aspect ratio (width to height ratio). These expressions let a parametric study of the problem based on which it is observed that the entropy generated due to flow in a duct of square cross-section is more than those of rectangular counterparts while increasing the aspect ratio decreases the entropy generation rate similar to what previously reported for the clear flow case
Heatline visualization of natural convection in a porous cavity occupied by a fluid with temperature-dependent viscosity
Temperature dependent viscosity effect in buoyancy driven flow, of a gas or a liquid, in an enclosure filled with a porous medium is studied numerically, based on the general model of momentum transfer in a porous medium. The Arrhenius model, which proposes an exponential form of viscosity-temperature relation, is applied to examine three cases of viscosity-temperature relation: constant, decreasing and increasing. Application of arithmetic and harmonic mean values of the viscosity is also investigated for their ability to represent the Nusselt number versus the effective Rayleigh number. Heatlines are illustrated for a more comprehensive investigation of the problem
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